The future demands for high-end CT and CV imaging regarding the X-ray source are higher power/tube current, shorter response-times regarding the tube current (pulse modulation) and smaller focal spots (FS) for higher image quality.
One key to reach higher power in smaller FS is given by using a sophisticated electron optical concept. But of same importance are the electron source itself and the starting condition of the electrons.
For today's high-end tubes directly heated thin flat emitters are used that are structured to define an electrical path and to obtain the required high electrical resistance. Basically, two different emitter designs comprising the explained features are well known: An emitter with a round or rectangular emitting surface/emitting section.
The first of the two types, for example explained in U.S. Pat. No. 6,426,587 B, is a thermionic emitter with balancing thermal conduction legs. The second type is explained later on. Both types have in common that they are directly heated thin flat emitters and that both emitter designs use slits to create an electric current path.
Generally, these types of emitters have a small thermal response time due to their small thickness of a few hundred of micrometers and sufficient optical qualities owing to their flatness. Variations of such designs are implemented in today's state-of-the-art X-ray tubes.
For directly heated electron sources it is essential that electrical resistance of the emitter and supplied current fulfill a required relation to release the necessary power within the filament following the equation for the powerP=I·R2  (1)
To achieve high power it is possible to apply high current or to increase the electrical resistance of the emitter. The last way may be realized with the known emitter of U.S. Pat. No. 6,426,587 B1.
The advantage of the emitters of the aforesaid types is that the entire electrical path can be realized with thin wires and narrow slits, resulting in a small device which is optimal for medical X-ray tubes. The disadvantage however may also based on the structuring: The electrical field may penetrates into the slit and the potential lines therefore bend into the slit region. If an electron is emitted from the surface perpendicular to the optical axis but within the region of deformed potential, its tangential velocity component may increases which causes stronger optical aberration of the source resulting in enlarged focal spots. An improvement of these known electron sources is essential.
Therefore, it is an object of the invention to provide an emitter which enables to get still smaller focal spot sizes while using today's sophisticated electron-optical lens systems.